Self-adaptive method for regulating an effluent treating plant, in particular for waste water

ABSTRACT

The invention concerns a method for regulating a wastewater or sludge treating plant. Ponds and cells provided with aerating means are used for eliminating carbonaceous, nitrogenous, and phosphate pollutants. The invention is characterized in that an automaton uses periods during which the plant is underloaded to impose on it abnormal operating conditions. The automaton analyses the response of the plant to the abnormal operating conditions to simulate and optimize the plant operating parameters for heavy-load periods, by automatically adjusting the parameters of the automaton logic.

The present invention relates to a self-adjusting method for regulatingeffluent treatment plants, in particular for wastewater or sludge whichresults from this type of treatment. The invention can in particular beapplied to such treatment plants in which elimination of variouspollutants, in particular carbonaceous, nitrogenous or phosphatepollutants, is provided. It is known that these plants use treatmentponds or cells equipped with aerating means and that they generallycomprise an aeration regulating system designed so as to make itpossible to obtain pollutant elimination yields corresponding to themaximum which can be achieved by the capacity of the plant.

Examples of implementation of such systems of regulation are describedin FR-A-2 724 646 and 2 765 210.

In these treatment plants, a physicochemical dephosphatation is carriedout, for which use is made of reagents such as in particular ferricchloride, which leads to a modification of the values for the redoxpotential of the wastewater in the process of being treated. This typeof interference is prejudicial to any system of regulation which takesinto account the evolution of the redox potential for managing theaeration of the treatment ponds, as is the case of the systems describedin the abovementioned publications. In this case, the thresholds of theoriginal logic of the system of regulation are unsuitable for thetreatment to eliminate the nitrogenous pollutants (nitrification anddenitrification) and it is then necessary to readjust these values.

Specifically, the presence of a reagent for simultaneous physicochemicaldephosphatation in the aerated sludge of wastewater treatment plantsleads to a modification of the redox equilibria. These reagents arepowerful oxidants and their addition to the sludge (for example in theform of ferric chloride or of aluminum salts) engenders an increase inthe redox values obtained, which is variable depending on the amount ofreagents injected into the sludge. In addition, it appears, in practice,that the redox values increase in a manner which is difficult to assess,depending on the reagent used and the operating conditions of the plant(for example modification of the load or of the rate of treatment,breakdown of the reagent metering pump). Such a sensitivity to thechanging of the operating conditions while running requires regularadjustment of the thresholds of the logic of the system of regulation.The generalization of wastewater dephosphatation treatment, required bythe national and European regulations, leads to the repeated occurrenceof this type of problem in existing purification plants, and the presentinvention has set itself the objective of providing a solution thereto.

With this aim, it appeared, to the present proprietor, that, initially,it was necessary to link the reference redox values to operatingparameters able to be measured on site, and to define new thresholdvalues as a function of the running conditions associated, for example,with the simultaneous physicochemical dephosphatation. Next, it appeareddesirable to develop an automated logic making it possible to vary thepredefined redox thresholds in the logic of the system of regulation, inorder to be free of manual interventions for re-setting the parametersof the thresholds of the logic for managing the treatment plant (inparticular the aeration) for any modification of the running conditions.It must be possible for this self-adjustment of the redox thresholds bythe automated regulating device to take place whatever the type ofreagent used and whatever the plant operating conditions. This shouldmake it possible to ensure the efficiency of the treatment of thenitrogenous pollutants (nitrification and denitrification) in along-lasting manner whatever the running context.

Another constraint which has had to be taken into account in theinvention is that of avoiding the further addition of material to thetreatment plant, in order to be able to use the method which is thesubject of the invention on systems of regulation implemented inaccordance with the prior technique mentioned above.

Consequently, the present invention relates to a method for regulatingan effluent treatment plant, in particular for wastewater or sludgewhich results from this type of treatment, using in particular ponds orcells equipped with aerating means, especially for eliminatingcarbonaceous, nitrogenous and phosphate pollutants, and an automatedregulating device operating on analysis of the changing of predeterminedoperating parameters of the plant, this method being characterized inthat:

-   -   the automated device uses periods during which the plant is        underloaded to impose on it abnormal operating conditions, and    -   the automated device analyzes the response of the plant to these        abnormal operating conditions so as to update and optimize the        plant operating parameters for heavy-load periods, by        automatically adjusting the parameter set-up of the automated        device logic.

Thus, the method which is the subject of the invention provides areadjustment of the threshold values of the reference redox potentialwhich are used in the automated regulating device according to the priortechnique specified above, so as to maintain guaranteed results fortreatment of the carbonaceous and nitrogenous pollutants in the presenceof simultaneous physicochemical dephosphatation. The operating of thislogic enables the automated device to automatically adapt to all runningconditions.

Thus, according to the invention, the maximum oxidation level attainedby the sludge in the aeration pond is determined in order toautomatically adjust the predefined thresholds in the logic of theautomated regulating device.

According to one embodiment of the invention:

-   -   the values of the redox potential are recorded during an        aeration forcing period, this period corresponding to a period        during which the plant is underloaded;    -   the maximum level of the redox potential, and also the        stabilizing thereof resulting in the presence of a redox        plateau, are detected;    -   the value of this redox plateau is measured and is stored;    -   the operation is recommenced over a given period of time, for        example of the order of one week;    -   the mean of the redox plateau over the periods in which the        latter is measured is calculated, and    -   said mean is used to obtain an updating of the redox potential        thresholds, by comparison with the redox potential thresholds of        origin, the automated device then operating using these new        thresholds.

A detailed description will now be given of an embodiment of the methodwhich is the subject of the invention, used for a treatment plantcomprising physicochemical dephosphatation simultaneously withtreatments to eliminate carbonaceous and nitrogenous pollutants. Thus,as will be explained below, this is merely a nonlimiting example ofimplementation, the method according to the invention being capable ofbeing put to other uses.

The conditions for forcing aeration during a period in which thewastewater treatment plant is underloaded, the conditions for recordingthe forcing value, the conditions for calculating the mean of the valuesthus obtained, the comparison between the mean thus calculated and theoriginal redox thresholds, and the permitted shifting of the redoxthresholds of the logic of the automated regulating device have beendescribed below.

Conditions for Forcing and Stopping the Aeration

The aim of forcing the aeration is to cause the redox to evolve at highvalues and to detect a stabilizing thereof during the nighttime period,characteristic of underloaded periods.

In this example of implementation of the method of the invention, theforcing of the aeration corresponds to the running, for a maximum periodof 2 hours, of all the available aeration means during the nighttimeperiod between 2 a.m. and 8 a.m. The beginning of the forcing shouldcorrespond to the first start-up on a regulating cycle between 2 a.m.and 8 a.m., maximum aeration is then demanded. There can be only onesingle forcing during the period considered.

The maximum aeration time permitted here will be 2 hours. If after 2hours the redox has not stabilized, the aeration will stopautomatically. The stopping of the aeration after the forcing willcorrespond to the system of regulation being re-operated, i.e. a maximumstop period of 2 hours.

In accordance with the method which is the subject of the invention, theforcing of the aeration should take place during periods ofunderloading. The nighttime slot of 2 a.m. to 8 a.m. chosen in theexample of implementation described here is representative of theconditions associated with municipal wastewater. The period of forcingcan be modifiable in precise contexts (in the case of regular nightlydischarge), without, however, departing from the context of theinvention.

Detection of a Redox Plateau During Forcing

The detection of what can suitably be called a redox plateau (definedbelow) will serve to determine the maximum level of oxidation of thesludge attained in the presence of the dephosphating reagent.

The redox value recorded for detecting the redox plateau will be themean over one minute of the values examined by the automated device, inorder to limit the effect of variation of the redox.

In this example of implementation, the stabilizing of the redox iscontrolled in the following way: during the maximum 2 hours of aeration,if the redox does not increase or does not decrease by more than 20 mVduring a 15 min period, then the aeration stops. These conditionscorrespond to the appearance of a stabilization corresponding to what isreferred to as the “redox plateau”. The maximum value recorded duringthe 15 min of stabilization will be memorized and stored in theautomated device. This value will correspond to the redox plateau of dayD.

If it has not been possible to detect a redox plateau, then the forcingof the aeration will end after 2 hours. No forcing redox value will bememorized for that precise day.

Conditions for Calculating the Mean of the Redox Forcings

Calculation of the mean of the redox forcings will serve to absorb thepossible fluctuations in these values, in such a way that action istaken in a time representative of the inertia of the system for mixingsludge and reagent.

A calculation of the mean of the redox forcing values recorded will berequested every 7 days, in this example of implementation. For this,conditions should be satisfied before allowing the automated device toaverage the values.

These conditions concern essentially the number of values:

-   -   if the number of forcing values recorded (plateaus reached) over        the 7 days is greater than or equal to 4, then the mean of the        values can be calculated;    -   conversely, if the number of forcing values is strictly less        than 4, then the mean of the values has no meaning and should        not be calculated. The mean will not be updated and will remain        the same as the preceding week. This lack of new value for the        mean will be recorded.

The mean forcing value for the week will be used for the comparison withthe ongoing threshold values of the system of regulation, so as todetermine their relevance in the updated running conditions.

If it has not been possible to calculate the weekly mean, this willindicate a considerable break in the measurement, which may be relatedto a problem of the redox electrode, consecutive unusual nighttimeevents, etc.

Use of the Weekly Mean for the Redox Forcings

The averaged value obtained will be compared to the ongoing upperthreshold value 2 (see table below) in the logic of the system ofregulation. The upper threshold 2 in fact corresponds to the maximumvalue which it is possible to attain when the oxidation of the sludge issatisfactory on the plant. This comparison will be used to determine therelevance of the thresholds used, and to modify them if this is provedto be necessary. This comparison, which is used to update thethresholds, will be carried out, for example, once a week.

The weekly averaged value will be used in the following way:

-   -   If present upper threshold 2—averaged value >+25 mV, then all        the redox thresholds of the logic of the system of regulation        are decreased by a level.    -   If present upper threshold 2—averaged value <−25 mV, then all        the redox thresholds of the logic of the system of regulation        are increased by a level.    -   If present upper threshold 2—averaged value <+25 mV and >−25 mV,        then no action is requested and the system of regulation        continues to operate with the thresholds preceding this        calculation.

These comparative tests will make it possible to determine thedifference existing between the values of the thresholds of the logic ofthe system of regulation used and the current redox values. An updatingof the thresholds will be requested if the calculated difference exceeds25 mV. If the difference is greater than +25 mV, then a decrease in thethresholds is requested; conversely, if the difference is greater than−25 mV, then an increase in the thresholds is necessary.

Request for Shift in the Thresholds of the Logic of the System ofRegulation

The thresholds will change according to the criteria of a predefinedtable. This table will make it possible to cause the redox thresholds tochange in steps of 25 mV for most of the thresholds of the system ofregulation, where appropriate in steps of 50 mV for the lower thresholdfor reinitiation. The table below gives the shifting of the thresholds.

TABLE Change in the redox thresholds of the logic of the system ofregulation Level of shift n − 3 n − 2 n − 1 n n + 1 n + 2 n + 3 Shifts(mV/H2) for +25 +25 +25 +25 +25 +25 thresholds 1-2-3 mV mV mV mV mV mVThreshold 2 450 475 500 525 550 575 600 400 425 450 475 500 525 550Threshold 1 350 375 400 425 450 475 500 300 325 350 375 400 425 450Threshold 3 200 225 250 275 300 325 350 Lower threshold 100 150 200 225250 275 300 Lower threshold +50 +50 +25 +25 +25 +25 shift mV mV mV mV mVmV

This table indicates a shifting of the redox thresholds in steps of 25mV for thresholds 1, 2 and 3 and, for most cases, for the lowerthreshold of the logic of the system of regulation. A single shift stepis permitted after each calculation of the weekly mean. These stepscorrespond to a slow evolution of the system and indicate a change,which is in spite of everything significant, in the redox equilibriumwithin the activated sludge of the plant.

The permitted threshold changes are limited to a maximum of 600 mV,corresponding to very high redox measurements rarely attained onpurification plants. Beyond this threshold, no increase is permitted andan alarm informs that this threshold has been exceeded. In the lowvalues, the redox thresholds are limited by the original thresholds ofthe logic of the system of regulation (level n-3). This makes itpossible to deal with a stopping of the injection of dephosphatingreagent, taking into account the inertia of the system. In fact,approximately one to three months are necessary for the completedisappearance of dephosphating reagent in the sludge (inertia relatingto more than three generations of sludge). The decrease in steps of 25mV will thus make it possible to gradually attain the thresholds of thelogic of the system of regulation. This minimum threshold correspondingto the threshold of the logic of the system of regulation makes itpossible to manage the disappearance of dephosphating reagent and tocontinue to control the aeration on known bases, without putting theautomated regulating device on the wrong track.

A product such as the dephosphating reagents acts as a strong oxidant onthe redox electrode. This effect is especially predominant at the lowredox values corresponding to the recommencement of aeration. For this,a further increase in the redox threshold for reinitiation, compared tothe other thresholds, is necessary in order to limit the aeration stoptimes. The lower threshold was defined at 225 mV for the reference shiftlevel n. Since an even step of 25 mV cannot include all the shifts, astep of 50 mV was defined between the thresholds 2 of 400-450 mV, inorder to be able to attain the lower threshold corresponding to thethreshold predefined in the logic of the system of regulation.

In order to facilitate the bringing into service and the assessmentwhich may ensue therefrom, the shift n represented in bold in the tableabove will correspond to the redox values at the time of theinitialization of the logic. In fact, the sites encountered until now,with conditions of simultaneous physicochemical dephosphatation, wereoperating with maximum redox values of approximately 500 mV/H2 in aperiod of aeration. This makes it possible to attain the limits of theredox thresholds established in the shift table, within a maximum periodof three weeks.

For practical reasons relating to the writing of the program, the valueslisted in the table above will be theoretical values. In fact, all thethreshold values of the logic of the system of regulation ending inother than 0 or 5 will be accepted. The programing of upper threshold 2and lower threshold 2 will lead to the automatic updating of the otherthresholds of this logic. In this way, it will be possible to have allunits between 0 and 5, but always with a constant differential betweenthe various thresholds corresponding to said logic.

Initialization of the Parameters

Before the automated device is brought into service, it is necessary toset up the parameters for the initialization values. The maininitialization values concern the redox thresholds of the logic of thesystem of regulation at the start-up of the program. These thresholdsshould be at the level n described in the table above.

Threshold 2 525 mV/H2 475 Threshold 1 425 mV/H2 375 Threshold 3 275mV/H2 Lower threshold 225 mV/H2

The intermediate level of these thresholds in the shift number makes itpossible to start up the automated device under any initial redoxconditions. In addition, this level corresponds to the thresholdscommonly encountered on physicochemical dephosphatation sites. One weekis sufficient for the automated device to place itself at 25 mV greateror lesser, and three weeks are necessary to attain the limits imposed.

The example of implementation above related to the adjusting of thepredefined thresholds in the logic of the automated regulating device inthe case of sites with simultaneous physicochemical dephosphatation. Inthis adjustment, the automated device uses periods in which the plant isunderloaded to impose on it abnormal operating conditions (here, theautomated device forces the aeration during a period of the day whenthis is not necessary). The automated device analyzes the response ofthe plant to these abnormal operating conditions in order to adjust itsoperating parameters. It will thus have a perfect parameter set-up toget the best out of the plant during heavy-load periods.

Thus, according to the method which is the subject of the invention, theautomated device uses periods during which the plant is underloaded toadjust its parameter set-up with a view to heavy-load periods.

The idea which forms the basis of the present invention thereforeconsists in using a reactor or a machine, in a period of underloading,to optimize the parameters thereof for periods of overloading. In theexample of implementation described above, it was used to control theaeration of a purification plant. Other uses are possible without,however, departing from the context of the invention. Some examplesthereof will be given below.

-   -   Control of the aeration of plants which are underloaded or which        have a variable load: in these two running types, the operating        of systems of regulation using redox potential thresholds can        pose problems. These thresholds are characteristic of a state of        the plant for a given load. If this load changes, the values of        the thresholds must be adjusted. Similarly, thresholds        programmed for a usual load must be modified when automatic        action is installed on a plant with a very low load. The device        used above automatically performs this adjustment of the redox        thresholds of the automatic action.    -   Optimization of flocculant metering in sludge treatment: this        is, for example, the case of units where a thickening screen is        coupled to a band filter. The mass flux of sludge to be treated        can be limited for a few moments, while at the same time varying        the flow rate of flocculant and measuring the flow rate of water        drained by the thickening screen. It is thus possible to adjust        (taking into account the quality of the sludge to be treated)        the flow rate of flocculant compared to the flow rate of water        drained by the thickening screen. During this operation, the        screen does not operate optimally. This deficiency must be        compensated by the band filter, and this is entirely possible        given the limitation of the mass load to be treated in this        phase.    -   Methods are, moreover, known for measuring the total amount of        sludge contained in a biological treatment plant for wastewater.        In this regard, reference may be made to FR-A-2 769 305. This        technique can be applied to plants in which the level of load is        sufficiently low to produce an effluent whose quality is        compatible with the European regulations. For these levels of        load, normal running of the plant automatically implies that the        mass of biomass contained in the treatment plant clarifier is        negligible during a period of hydraulic underload (usually at        night). This condition is necessary and sufficient for it to be        possible to use this method of measuring the total mass of        sludge contained in a plant.

The present invention makes it possible to extend the field ofapplication of the method of measuring the total mass of sludgecontained in a plant in accordance with the French publication mentionedabove FR-A2 769 305.

When, even during a period of hydraulic underload, the mass of biomasscontained in the clarifier is not negligible, the present invention canbe used, imposing maximum values for the recirculation flow rate. Thisbehavior, unusual among those skilled in the art, will bring aboutemptying of the clarifier and will make it possible to measure the totalmass of sludge contained in the plant.

-   -   The aeration ponds in purification plants are usually stirred.        The aim of this stirring is to homogenize the concentration of        the materials in these ponds. Those skilled in the art        therefore, as far as possible, keep this stirring system        functioning. It is, in addition, necessary (see FR-A-2 784 093),        in a system of automation, to measure the ability of the sludge        to settle out. This measurement is performed using a manual test        of settling out in a test tube. The present invention makes it        possible to avoid this manual measurement.

It is necessary, for this, to stop the device during a period ofnonaeration, for a given limited amount of time (for example one hour),and then to monitor the concentration of the materials in suspension ata given depth of the pond (for example 50 centimeters). Analysis of thiscurve gives those skilled in the art a measure of the ability of thesludge to settle out. In order to avoid any incorrect operation of theplant, it is necessary to carry out the stopping of the stirring duringperiods when the treatment unit is underloaded.

It remains, of course, that the present invention is not limited to theexamples of implementation and/or of use described and representedabove, but that it encompasses all the variants thereof.

1. A method for regulating an effluent treatment plant, in particularfor wastewater or sludge which results from this type of treatment,using in particular ponds or cells equipped with aerating means,especially for eliminating carbonaceous, nitrogenous and phosphatepollutants, and an automated regulating device operating on analysis ofthe changing of predetermined operating parameters of the plant,characterized in that: the automated device uses periods during whichthe plant is underloaded to impose on it abnormal operating conditions,and the automated device analyzes the response of the plant to theseabnormal operating conditions so as to update and optimize the plantoperating parameters for heavy-load periods, by automatically adjustingthe parameter set-up of the automated device logic.
 2. The method asclaimed in claim 1, used in particular for the simultaneous treatment ofcarbonaceous, nitrogenous and phosphate pollutants, characterized inthat the automated device forces the aeration during a period in whichthe plant is underloaded, and it analyzes the response of the plant tothese abnormal operating conditions in order to adjust the plantoperating parameters, optimizing them for periods of overload.
 3. Themethod as claimed in claim 2, characterized in that: the values of theredox potential are recorded during an aeration forcing period, thisperiod corresponding to a period during which the plant is underloaded;the maximum level of the redox potential, and also the stabilizingthereof resulting in the presence of a redox plateau, are detected; thevalue of this redox plateau is measured and is stored; the operation isrecommenced over a given period of time, for example of the order of oneweek; the mean of the redox plateau over the periods in which the latteris measured is calculated, and said mean of the redox plateau is used toobtain an updating of the redox potential thresholds, by comparison withthe redox potential thresholds of origin, the automated device thenoperating using these new thresholds.
 4. The method as claimed in claim1, used for flocculant metering in sludge treatment, in particular onunits using a thickening screen coupled to a band filter, characterizedin that: the mass flux of sludge to be treated is limited for a givenperiod of time, while at the same time varying the flow rate offlocculent; the flow rate of water drained by the thickening screen ismeasured; and the flow rate of flocculant is adjusted compared to theflow rate of water drained by the thickening screen.
 5. The method asclaimed in claim 1, used for measuring the total mass of sludgecontained in a biological purification plant, characterized in that,during a period of hydraulic underload, maximum values for the sludgerecirculation flow rate are imposed, which brings about emptying of theplant's clarifier, and the total mass of sludge contained in the plantis measured.
 6. The method as claimed in claim 1, used for measuring theability of the sludge to settle out in a purification plant comprisingstirring of the aeration ponds in order to ensure homogenization of theconcentration of the materials in suspension in the sludge,characterized in that: the stirring is stopped during the periods ofunderload; the change in concentration of the materials in suspension ata given depth of the aeration pond is monitored; the curve obtained isanalyzed, and the ability of the sludge to settle out is deducedtherefrom.